This invention relates to cleaning apparatus of the sootblower type employed to direct jets of air, steam, water, or a mixture of such agents against fouled or slag-encrusted components of large scale boilers and other heat-exchangers typically used by public utilities and in industry for the production of steam for power generation and other purposes. (The term "boiler" is intended to encompass other heat-exchangers to which this invention is applicable.) The invention relates particularly to sootblowers of the retracting type, wherein the cleaning jets are moved into the boiler to clean and upon completion of their cleaning cycle, are then withdrawn from the severe environment therein. Sootblowers of this type employ a retracting lance tube typically having two or more radially directed nozzles near the outer end.
In order to equalize the jet reaction forces on the cantilevered lance tube when it is in operation in the boiler, the nozzles are oppositely or equally spaced peripherally and their axis intersects the longitudinal axis of the lance tube. In order to permit the lance tube to move into and out of the boiler through the substantially sealed and/or air-shielded opening in the wall box, the nozzles must, as a practical matter, be located entirely within the lance tube. Due to the restricted diameter of the lance tube and the volume of blowing medium normally required for effective cleaning and/or to adequately cool the lance while it is in the boiler, it has in many instances been impossible to provide opposing nozzles having optimal dimensions for the production of a concentrated high velocity jet that is desired for efficient cleaning.
As a sootblower lance is inserted into and retracted from the boiler, it is simultaneously rotated and/or oscillated about its longitudinal axis so that the blowing medium jet sweeps a helical or partially helical path. The lance typically rotates a number of times during its projection and retraction movement. Since the speed at which the lance may safely be rotated is limited by the critical speed above which the lance becomes dynamically unstable, the total cycle time required to insert and retract the lance becomes restricted by this consideration. Therefore, for some applications, the cycle time of a sootblower must be made greater in duration than dictated by cleaning requirements. In many instances, particularly where high combustion gas temperatures or wide boilers are involved, a certain minimum flow of blowing medium must be maintained in order to provide sufficient cooling to protect the lance tube in this severe environment, resulting in a considerable waste of blowing medium. Moreover, longer sootblower cycle times lead to additional power consumption and component wear.
Fluidic pressure of blowing medium acting on the lance tube exerts a projecting force on the lance which resists lance retraction, thereby requiring considerably more energy to retract the lance than to insert it. Reduction in retraction load would result in reducing power consumption and would decrease component mechanical loading.
This invention is directed to addressing the above-mentioned shortcomings and design concerns of prior art sootblowers of the retracting type.
One of the objects of this invention is the provision of improved lance tube designs which permit the use of more efficient nozzle configurations thereby enhancing the sootblower cleaning performance. A further object is to reduce the number of lance rotations necessary to achieve a desired jet path spacing. A still further object of the invention is to provide means for partially counteracting the rotational component of the lance pressure force acting to cause lance insertion and acting against lance retraction. Another object of this invention is to provide a long retracting sootblower design which features improved efficiency in terms of blowing medium consumption during cleaning.
It has been common practice in the prior art to employ two or more nozzles at one longitudinal position of the lance of a long retracting blower. With the large volume of blowing medium required for lance cooling and adequate cleaning, these configurations lead to short relative nozzle lengths which results in high turbulence and rapid dispersion of the discharged blowing medium. Additionally, the close proximity of the inlets of nozzles to one another further introduces turbulence and restriction to flow.
The ratio of the nozzle length to its throat diameter is an important parameter in establishing the nozzle flow condition, generally the larger the ratio the less turbulent the jet from the nozzle, which produces a more concentrated jet stream thus achieving greater impact pressures at a given distance for a given flow rate. By placing nozzles at different longitudinal positions so they are not directly opposite each other, greater nozzle lengths and a greater number of nozzles may be employed, improving the ratio of the length of the nozzle to the throat diameter. Further, by spacing the nozzles such that their centerlines are not colinear, each may project further into the lance tube such that the fluid flow into each is minimally obstructed by other nozzles, thereby reducing restriction and turbulence. By placing a plurality of nozzles in the lance tube at different longitudinal positions along the lance, an important additional benefit is realized. Such a configuration enables the ratio of rotational travel to longitudinal travel of the lance to be reduced while maintaining a desired cleaning effect. As will be shown, the number of lance rotations necessary to produce a desired pitch spacing between spray paths is inversely related to the number of different lance longitudinal positions where nozzles are placed and the number of nozzles at those locations. A reduction in rotational velocity to longitudinal velocity correspondingly enables shorter cycle times before lance dynamic instability becomes a problem.
A further object of this invention is to provide an improved lance having opposing nozzles which are offset such that their longitudinal axes do not intersect the lance tube centerline. The offset mounting is such that longer, more efficient nozzles may be used to produce higher jet impact pressures than otherwise would be obtainable, and, further, a thrust reaction couple is generated which acts upon the lance in a retracting direction. Since the lance rotation and longitudinal movement are related by a gear drive within the blower carriage mechanism, the applied torque causes a longitudinal force on the lance. By causing nozzle thrust to oppose the direction of rotation of the lance on insertion, the tendency for the lance to be projected into the boiler on carriage "runaway" is at least partially offset. Conversely, the nozzle thrust aids in retraction since the direction of rotation is reversed. Since the peak lance drive loads occur upon retraction, this improvement permits the use of more efficient drive systems.